Reorganize test helpers

[kidder]

Feature request:

Our test helpers are scattered, poorly documented, and incomplete. We should do a better job.

Component:

• [ ] Code
• [x] Documentation
• [ ] Build system
• [x] Continuous integration

Desired feature:

• [x] Test helpers are moved into their own directory so they can be found and easily documented
• [ ] It would be useful to have TestTags and TestItems that can be included instead of duplicated in multiple tests
• [ ] Make sure TestHelpers are properly documented and show up in the Doxygen (as we exclude test directories by default)
• [ ] Split monolithic test helper files into smaller ones.

Detailed discussion:

Here are some initial thoughts (with @nilsdeppe).

• Introduce dir tests/Unit/Helpers inside which we would have files such as Serialization, MoveSemantics, NonCopyable, Comparators, Iterators, ...
• Move current test helpers from specific dirs such as tests/Unit/Domain/CoordinateMaps/TestMapHelpers.hpp to subdirs in test/Unit/Helpers, and splitting them into smaller files which are compiled into libs that can be linked
• Make the TestHelpers namespace standard, and create a TestHelpers::Tags namespace that holds commonly used test tags.
• Move files in tests/Utilities/ to test/Unit/Helpers

[kidder]

@wthrowe do you have any thoughts on this?

[nilsdeppe]

@kidder has any of this been addressed? I notice we have the tests/Unit/Helpers directory now